ABSTRACT
Positive feedback In Chapter 7, we showed how negative feedback can be applied to an amplifier to form the basis of a stage which has a precisely controlled gain. An alternative form of feedback, where the output is fed back in such a way as to reinforce the input (rather than to subtract from it), is known as positive feedback. Figure 9.1 shows the block diagram of an amplifier stage with positive feedback applied. Note that the amplifier provides a phase shift of 180° and the feedback network provides a further 180°. Thus the overall phase shift is 0°. The overall voltage gain, G, is given by:
By applying Kirchhoff’s Voltage Law
thus
and
where Av is the internal gain of the amplifier. Hence:
Now consider what will happen when the loop gain, 0Av, approaches unity (i.e., when the loop gain is just less than 1). The denominator (1 2 0Av) will become close to zero. This will have the effect of increasing the overall gain, i.e. the overall gain with positive feedback applied will be greater than the gain without feedback.
